The present invention relates to shaping fiber preforms by winding a fiber texture that has been obtained by three-dimensional (3D) or multilayer weaving. Such fiber preforms constitute fiber reinforcing structures for composite material parts.
The invention relates more particularly to fiber preforms that are to form the reinforcement of axisymmetric composite material parts that present a profile in section that varies (varies in shape and/or thickness), such as for example an aeroengine fan casing. The fiber preform for such a part is made by weaving a fiber texture and winding that texture under tension onto an axisymmetric mold including a winding surface of radial section that includes a portion in relief corresponding to the profile of the part that is to be fabricated.
In order to make a fiber texture that is adapted to the varying shape of the injection mold, use is made of shape weaving, also referred to as “contour weaving” or “outline weaving”, which consists in taking up different lengths of warp yarns as a function of their position across the width of the fiber texture that is being woven in the form of a strip, so as to obtain ratios between the warp yarn lengths (tangential direction) that are similar to the ratios between the radii defining the varying profile of the final part that is to be fabricated.
The fiber texture as woven in that way is stored on a storage mandrel for subsequent winding under tension onto an axisymmetric injection mold that includes a winding surface having in radial section a profile in relief that corresponds to the profile of the part to be fabricated. While the texture is being wound onto the mold, it is put under tension by the storage mandrel, which is also referred to as the “takeup” mandrel, with one or more follower rollers being placed between the storage mandrel and the injection mold. The follower roller(s) present(s) radii that vary across their axial width so as to define an outside surface or profile in relief that serves, between the storage mandrel and the injection mold, to conserve the yarn length ratios as defined during weaving. Conserving yarn length ratios thus serves to maintain uniform tension in the fiber texture.
While the fiber preform is being shaped on the mold, the fiber texture is rolled up through a plurality of turns. For each turn the ratio between the various radii defining the profile in relief changes as a function of the layers of fiber texture that have already been wound. These changes in profile are themselves greater when the shape and/or thickness ratios in the final part are large.
While the fiber texture is being wound onto the injection mold, it can become offset because of the differences between the woven profile and the real profile onto which it is being wound, thereby leading to losses of tension at certain axial positions across the texture. These unbalanced tensions across the width of the part can lead to defects such as waves, fibers buckling, fibers being pinched, zones of unwanted extra thickness, and out-of-specification fiber volume fractions. These tension unbalances can also complicate the shaping of the fiber texture by winding by giving rise in particular to the formation of creases or of misalignments, thereby causing the shaping of the fiber texture to be more arduous and to take longer.